Burlington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burlington clear sky chart from 2005-11-06 to 2026-05-02.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	13% 9% 4 9% 6% 5% 5% 4 6% 7% 32%	131936
	Night	13% 9% 3 9% 6% 5% 5% 4 6% 7% 33%	130519
Transparency	Day	32% 37% 8% 14% 8% 1	135531
	Night	33% 36% 8% 14% 8% 1	135010
Seeing	Day	16% 8% 25% 28% 16% 6%	16271
	Night	18% 8% 28% 29% 14% 2	25851

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 11% 5% 12% 8% 7% 6% 4 5% 6% 24%	4661
	11th to 20th	Day	7% 17% 8% 19% 10% 8% 6% 3 4 5% 13%	5228
	21st to end	Day	13% 13% 6% 13% 8% 7% 6% 4 5% 5% 19%	5780
February	1st to 10th	Day	13% 15% 5% 11% 7% 7% 7% 4 5% 5% 22%	1890
	11th to 20th	Day	12% 12% 5% 13% 8% 6% 6% 4 5% 6% 23%	1843
	21st to end	Day	16% 12% 4 12% 6% 5% 5% 4 5% 6% 24%	1636
March	1st to 10th	Day	16% 9% 4 10% 7% 5% 5% 3 5% 6% 29%	5042
	11th to 20th	Day	9% 11% 5% 10% 6% 6% 6% 4 6% 8% 30%	5090
	21st to end	Day	14% 9% 4 9% 6% 6% 6% 4 6% 5% 32%	5586
April	1st to 10th	Day	16% 10% 3 9% 6% 6% 4 4 5% 6% 30%	2461
	11th to 20th	Day	15% 9% 3 8% 6% 5% 4 4 5% 6% 35%	2586
	21st to end	Day	17% 9% 3 8% 5% 5% 4 4 5% 6% 34%	2730
May	1st to 10th	Day	18% 8% 3 8% 4 4 4 4 5% 7% 33%	5901
	11th to 20th	Day	13% 10% 3 9% 6% 4 5% 4 6% 7% 31%	6176
	21st to end	Day	15% 7% 3 7% 4 5% 4 4 6% 7% 39%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	2800
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 7% 9% 39%	2800
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 6% 10% 41%	2800
July	1st to 10th	Day	8% 4 2 6% 4 5% 6% 5% 7% 11% 42%	6360
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 4 8% 9% 44%	6279
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 9% 48%	6424
August	1st to 10th	Day	15% 6% 2 6% 4 4 6% 4 7% 8% 38%	4234
	11th to 20th	Day	8% 7% 3 8% 5% 5% 6% 5% 8% 10% 36%	4174
	21st to end	Day	8% 6% 2 6% 5% 5% 6% 4 6% 9% 43%	4512
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	2400
	11th to 20th	Day	8% 6% 3 5% 4 4 4 3 6% 9% 48%	2255
	21st to end	Day	14% 6% 3 7% 6% 5% 5% 4 6% 8% 36%	2175
October	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 3 5% 7% 37%	2000
	11th to 20th	Day	17% 8% 4 9% 6% 4 4 4 6% 7% 31%	2000
	21st to end	Day	21% 9% 4 10% 6% 5% 6% 5% 5% 6% 25%	2200
November	1st to 10th	Day	11% 10% 3 8% 6% 5% 5% 6% 6% 8% 33%	1810
	11th to 20th	Day	12% 11% 3 10% 7% 6% 6% 5% 5% 7% 28%	1680
	21st to end	Day	17% 13% 5% 8% 7% 6% 5% 5% 6% 7% 22%	1680
December	1st to 10th	Day	15% 15% 5% 11% 8% 6% 6% 4 6% 6% 16%	2680
	11th to 20th	Day	17% 16% 4 10% 6% 5% 5% 4 5% 5% 23%	3066
	21st to end	Day	23% 16% 6% 13% 6% 6% 5% 4 5% 4 12%	4048
January	1st to 10th	Night	13% 12% 4 11% 7% 6% 5% 4 6% 7% 25%	8234
	11th to 20th	Night	9% 12% 5% 15% 10% 7% 6% 4 6% 7% 19%	7639
	21st to end	Night	13% 11% 5% 12% 8% 6% 6% 4 5% 7% 24%	8401
February	1st to 10th	Night	14% 14% 4 10% 7% 5% 6% 4 6% 6% 26%	2730
	11th to 20th	Night	12% 12% 4 11% 6% 5% 5% 5% 6% 6% 28%	2663
	21st to end	Night	13% 12% 4 10% 7% 6% 5% 4 7% 6% 27%	2166
March	1st to 10th	Night	15% 7% 3 8% 5% 5% 6% 5% 6% 7% 32%	5018
	11th to 20th	Night	10% 8% 3 7% 5% 5% 5% 4 6% 8% 39%	5089
	21st to end	Night	16% 9% 3 8% 5% 4 4 3 4 5% 38%	5588
April	1st to 10th	Night	16% 10% 3 9% 6% 5% 4 4 5% 6% 32%	2158
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 4 5% 7% 36%	2000
	21st to end	Night	14% 8% 3 6% 6% 4 4 4 6% 6% 38%	1890
May	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	4062
	11th to 20th	Night	13% 8% 3 8% 6% 5% 6% 5% 7% 7% 33%	3713
	21st to end	Night	16% 5% 2 6% 4 4 4 4 6% 8% 43%	3926
June	1st to 10th	Night	10% 5% 2 6% 4 5% 6% 6% 5% 8% 43%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 4 6% 4 8% 8% 43%	1600
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 5% 8% 46%	1600
July	1st to 10th	Night	6% 4 2 6% 4 6% 5% 4 6% 10% 48%	3600
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 6% 8% 49%	3664
	21st to end	Night	7% 5% 2 5% 4 4 4 4 7% 8% 51%	4428
August	1st to 10th	Night	9% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2915
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2875
	21st to end	Night	8% 6% 2 5% 3 5% 5% 4 7% 9% 47%	3352
September	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 5% 8% 46%	2000
	11th to 20th	Night	9% 4 1 4 3 3 4 3 5% 7% 55%	2142
	21st to end	Night	14% 7% 2 7% 5% 4 5% 4 6% 7% 38%	2225
October	1st to 10th	Night	16% 7% 2 6% 4 4 3 3 5% 6% 44%	2400
	11th to 20th	Night	17% 8% 3 7% 5% 5% 5% 3 6% 7% 35%	2400
	21st to end	Night	19% 9% 3 8% 4 5% 5% 4 6% 5% 32%	2640
November	1st to 10th	Night	14% 7% 3 7% 4 5% 4 3 6% 8% 39%	2703
	11th to 20th	Night	13% 10% 4 8% 6% 5% 6% 4 6% 8% 30%	2940
	21st to end	Night	17% 10% 4 8% 6% 5% 5% 5% 6% 7% 27%	2940
December	1st to 10th	Night	11% 14% 5% 10% 6% 6% 6% 5% 8% 8% 21%	4720
	11th to 20th	Night	15% 15% 4 9% 6% 6% 5% 4 5% 6% 25%	5348
	21st to end	Night	24% 16% 6% 12% 5% 5% 5% 3 4 5% 15%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	48% 22% 9% 12% 8% 1	4662
	11th to 20th	Day	60% 22% 5% 9% 4	5262
	21st to end	Day	53% 23% 8% 11% 5%	5780
February	1st to 10th	Day	26% 60% 4 7% 3	1890
	11th to 20th	Day	24% 59% 4 10% 2	1852
	21st to end	Day	22% 60% 6% 8% 4 1	1636
March	1st to 10th	Day	40% 31% 9% 10% 9%	5045
	11th to 20th	Day	35% 30% 14% 15% 5% 2	5090
	21st to end	Day	35% 31% 7% 17% 9% 1	5586
April	1st to 10th	Day	24% 60% 3 8% 5%	2461
	11th to 20th	Day	21% 60% 4 10% 5% 1	2586
	21st to end	Day	21% 60% 5% 9% 5% 1	2730
May	1st to 10th	Day	36% 30% 10% 11% 7% 5%	5910
	11th to 20th	Day	39% 32% 8% 11% 9% 1	6180
	21st to end	Day	34% 29% 7% 16% 12% 2	6949
June	1st to 10th	Day	16% 63% 4 9% 7% 1	2800
	11th to 20th	Day	17% 62% 5% 9% 6% 1	2800
	21st to end	Day	15% 62% 4 11% 9%	2800
July	1st to 10th	Day	28% 26% 10% 23% 12% 1	6290
	11th to 20th	Day	25% 25% 8% 28% 12% 2	6204
	21st to end	Day	23% 26% 11% 24% 16% 1	6240
August	1st to 10th	Day	29% 37% 9% 14% 11% 1	4221
	11th to 20th	Day	29% 37% 11% 17% 5%	4213
	21st to end	Day	21% 37% 6% 28% 8%	4512
September	1st to 10th	Day	16% 58% 7% 14% 6%	2400
	11th to 20th	Day	12% 59% 5% 15% 9%	2255
	21st to end	Day	17% 58% 7% 12% 5%	2175
October	1st to 10th	Day	22% 36% 9% 22% 10% 1	3260
	11th to 20th	Day	21% 36% 10% 19% 14%	3240
	21st to end	Day	31% 36% 10% 17% 6%	3538
November	1st to 10th	Day	26% 55% 6% 10% 3	1810
	11th to 20th	Day	23% 56% 6% 9% 5% 1	1680
	21st to end	Day	27% 56% 7% 6% 4	1680
December	1st to 10th	Day	44% 35% 12% 6% 2	2680
	11th to 20th	Day	44% 31% 9% 9% 7% 1	3066
	21st to end	Day	57% 29% 5% 5% 3	4048
January	1st to 10th	Night	45% 23% 9% 17% 5%	8234
	11th to 20th	Night	52% 23% 7% 12% 6%	7680
	21st to end	Night	49% 23% 7% 12% 8%	8401
February	1st to 10th	Night	24% 59% 5% 9% 2	2730
	11th to 20th	Night	22% 58% 5% 11% 4	2665
	21st to end	Night	23% 60% 6% 7% 4	2166
March	1st to 10th	Night	37% 29% 9% 13% 9% 2	5028
	11th to 20th	Night	28% 29% 18% 16% 8% 1	5090
	21st to end	Night	34% 31% 5% 15% 13% 1	5588
April	1st to 10th	Night	25% 59% 4 7% 4 1	2158
	11th to 20th	Night	21% 59% 4 10% 5% 1	2000
	21st to end	Night	18% 59% 5% 11% 6% 1	1890
May	1st to 10th	Night	42% 29% 8% 8% 8% 5%	4069
	11th to 20th	Night	39% 32% 9% 12% 8%	3714
	21st to end	Night	32% 30% 10% 17% 12%	3927
June	1st to 10th	Night	15% 62% 3 10% 8% 1	1600
	11th to 20th	Night	16% 62% 5% 10% 7% 1	1600
	21st to end	Night	16% 60% 3 10% 9% 1	1600
July	1st to 10th	Night	28% 25% 11% 18% 16% 2	3560
	11th to 20th	Night	26% 25% 9% 24% 15% 1	3616
	21st to end	Night	24% 25% 10% 21% 18% 1	4297
August	1st to 10th	Night	25% 36% 9% 18% 11% 2	2904
	11th to 20th	Night	28% 36% 9% 21% 6%	2902
	21st to end	Night	24% 35% 8% 23% 9%	3352
September	1st to 10th	Night	14% 57% 8% 13% 9%	2000
	11th to 20th	Night	11% 57% 6% 14% 11% 1	2142
	21st to end	Night	19% 57% 5% 13% 6%	2225
October	1st to 10th	Night	19% 36% 5% 20% 19%	3920
	11th to 20th	Night	27% 35% 8% 21% 9%	3899
	21st to end	Night	21% 36% 12% 22% 9%	4252
November	1st to 10th	Night	24% 54% 6% 9% 6%	2703
	11th to 20th	Night	22% 55% 6% 12% 5%	2940
	21st to end	Night	26% 55% 8% 8% 3	2940
December	1st to 10th	Night	42% 34% 12% 8% 3	4720
	11th to 20th	Night	42% 32% 6% 11% 9% 1	5348
	21st to end	Night	54% 28% 7% 8% 3	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 13% 33% 26% 8% 1	220
	11th to 20th	Day	23% 16% 38% 15% 7% 2	327
	21st to end	Day	20% 13% 37% 23% 5% 1	363
February	1st to 10th	Day	20% 12% 42% 19% 7% 1	330
	11th to 20th	Day	20% 19% 34% 24% 3	330
	21st to end	Day	22% 21% 33% 18% 5%	282
March	1st to 10th	Day	19% 10% 36% 26% 8%	540
	11th to 20th	Day	19% 15% 34% 25% 6% 1	540
	21st to end	Day	24% 10% 29% 28% 8% 1	594
April	1st to 10th	Day	18% 10% 33% 30% 8% 1	540
	11th to 20th	Day	21% 13% 26% 25% 10% 4	532
	21st to end	Day	21% 8% 27% 30% 12% 2	540
May	1st to 10th	Day	18% 8% 23% 31% 14% 5%	507
	11th to 20th	Day	16% 6% 21% 31% 21% 6%	527
	21st to end	Day	13% 4 20% 35% 21% 8%	573
June	1st to 10th	Day	12% 5% 26% 37% 17% 3	530
	11th to 20th	Day	11% 5% 19% 34% 26% 5%	530
	21st to end	Day	14% 3 20% 35% 22% 6%	530
July	1st to 10th	Day	7% 2 12% 36% 30% 13%	525
	11th to 20th	Day	12% 3 15% 32% 27% 11%	524
	21st to end	Day	8% 2 15% 38% 28% 9%	576
August	1st to 10th	Day	10% 3 15% 30% 28% 14%	526
	11th to 20th	Day	9% 3 17% 29% 28% 15%	519
	21st to end	Day	11% 3 17% 30% 26% 12%	583
September	1st to 10th	Day	13% 2 20% 29% 22% 14%	530
	11th to 20th	Day	8% 3 15% 26% 28% 19%	530
	21st to end	Day	15% 7% 22% 28% 20% 8%	530
October	1st to 10th	Day	18% 8% 22% 24% 18% 10%	530
	11th to 20th	Day	18% 11% 32% 26% 10% 4	530
	21st to end	Day	26% 9% 28% 25% 10% 1	583
November	1st to 10th	Day	15% 16% 31% 27% 9% 2	328
	11th to 20th	Day	21% 15% 28% 26% 8% 1	220
	21st to end	Day	21% 13% 38% 20% 8%	220
December	1st to 10th	Day	21% 15% 34% 22% 8%	220
	11th to 20th	Day	26% 22% 28% 21% 4	220
	21st to end	Day	25% 13% 41% 16% 5%	242
January	1st to 10th	Night	22% 14% 38% 20% 6%	840
	11th to 20th	Night	25% 15% 35% 20% 4	840
	21st to end	Night	17% 12% 41% 24% 6%	924
February	1st to 10th	Night	21% 13% 40% 22% 3	840
	11th to 20th	Night	20% 13% 38% 25% 4	823
	21st to end	Night	21% 13% 37% 25% 4	692
March	1st to 10th	Night	19% 8% 36% 28% 8%	840
	11th to 20th	Night	21% 11% 34% 26% 7%	840
	21st to end	Night	21% 8% 29% 32% 9% 1	924
April	1st to 10th	Night	21% 8% 31% 33% 6%	840
	11th to 20th	Night	21% 8% 31% 29% 11% 2	756
	21st to end	Night	19% 5% 26% 37% 13%	630
May	1st to 10th	Night	18% 3 24% 33% 18% 3	603
	11th to 20th	Night	18% 4 22% 38% 18% 1	454
	21st to end	Night	12% 3 21% 37% 26% 1	433
June	1st to 10th	Night	12% 2 24% 42% 18% 2	400
	11th to 20th	Night	12% 2 24% 38% 23% 1	400
	21st to end	Night	11% 3 19% 43% 25% 1	400
July	1st to 10th	Night	6% 1 15% 46% 28% 3	390
	11th to 20th	Night	10% 2 19% 38% 25% 5%	413
	21st to end	Night	9% 2 12% 47% 28% 3	657
August	1st to 10th	Night	9% 2 12% 40% 33% 5%	597
	11th to 20th	Night	9% 2 11% 37% 36% 5%	594
	21st to end	Night	11% 2 18% 35% 30% 5%	737
September	1st to 10th	Night	11% 4 19% 31% 27% 7%	800
	11th to 20th	Night	11% 4 17% 27% 34% 8%	800
	21st to end	Night	17% 5% 18% 31% 24% 5%	800
October	1st to 10th	Night	18% 6% 25% 25% 23% 4	800
	11th to 20th	Night	21% 8% 30% 29% 10% 2	800
	21st to end	Night	24% 9% 30% 30% 7%	880
November	1st to 10th	Night	19% 9% 29% 31% 10% 1	820
	11th to 20th	Night	20% 12% 35% 23% 9% 2	840
	21st to end	Night	23% 11% 35% 25% 6%	840
December	1st to 10th	Night	25% 14% 34% 20% 7%	840
	11th to 20th	Night	22% 16% 37% 21% 4	840
	21st to end	Night	29% 12% 34% 22% 3	924

Page generated at 2026-04-29 02:21:47. Version ac07

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